Communication cables are generally used to transmit a variety of signals, including voice, video, and data signals. Each cable typically contains a single or multiple strands of a transmission media (e.g., copper wire) coated with an insulating material. The strands of the insulated transmission media are generally contained within a hollow core of a tubular jacket. The insulating material, often called the insulation, confines the signals to the transmission media during transmission. While the jacket can be electrically insulating, its main purpose is to provide mechanical and environmental protection.
In addition to exhibiting many other characteristics, it is often necessary that communication cables exhibit fire resistance. When installed in buildings, communication cables are often routed through the ductwork of the building's air exchange system. Such cables are called plenum cables. To reduce the risk of fire propagating through the building's ductwork, safety codes often require that plenum cables meet industry standards for low smoke generation and low flame spread.
One type of plenum cable often used in voice and data communication systems of commercial buildings is an UTP (unshielded twisted pair) cable. There are several industry standards for these types of cables, including the NFPA 262 requirement and the electrical transmission requirements as defined in ANSI/TIA/EIA 568-B.2. Generally, UTP types of cable contain four individually twisted wire pairs comprised of 24 AWG copper conductors. Each wire is individually insulated with an insulation material.
Conventional insulation materials include thermoplastic polymers that are solid at operation temperatures, but soften and flow upon application of heat and pressure. The most common thermoplastic polymer in plenum cables is fluorinated ethylene-1-propylene copolymer (FEP). See, for example, U.S. Pat. Nos. 5,841,072, 5,841,073, and 5,563,377, the disclosures of which are incorporated herein by reference.
Current plenum cable designs have 3 general types of constructions. First, the cable insulation contains only FEP and is expensive to produce due to the cost of FEP. Second, the cable is a composite of FEP and other materials. See, for example, U.S. Pat. Nos. 5,841,073, 5,932,847, and 5,841,072, the disclosures of which are incorporated herein by reference. Third, the cable replaces the FEP with other polymers, but modifies the jacket of the cable significantly to compensate for the loss of insulation properties from the FEP. See, for example, U.S. Pat. No. 6,392,152 the disclosure of which is incorporated herein by reference.
Unfortunately, none of these conventional insulating materials or configurations is completely satisfactory. For example, FEP materials are quite expensive. Designs consisting of a composite of FEP and non-FEP materials often require twist length or expansion consideration to minimize signal propagation delay skew. As well, producing a cable with multiple insulation constructions increases manufacturing complexity and product cost. Using jacket materials to compensate for the reduced fire performance of the insulated conductors is also an expensive alternative. Typically, jackets used in such constructions are PVDF or PVC/PVDF alloys.